1. Field of the Invention
This invention relates to the field of sealing systems for air heaters and more particularly to sealing systems at the circumferential interfaces between the rotatable and stationary elements of rotary regenerative heat exchangers.
2. Discussion of the Related Art
It is typical in fuel burning installations, such as electrical power generating plants, to use rotary regenerative air heaters for preheating the intake air to improve the efficiency of the fuel burning operation. These air heaters typically include two main elements or components, the first main element being a generally cylindrical drum, which carries a matrix of heat exchange elements, called baskets; the second main element being a housing having a generally cylindrical opening, in which housing the drum is located.
In the most common type of air heater, the Ljungstrom type, the drum takes the form of a rotor and the housing is stationary, such that the drum or rotor is rotatably mounted in the stationary housing. Another type of air heater uses a stationary drum containing the heat exchange elements and a movable housing surrounding the drum. This type of air heater is known as a Rothermuhle type air preheater, which is exemplified in U.S. Pat. 3,802,489 assigned to Apparatebau Rothermuhle Brandt & Kristzler of Wenden, West Germany, the disclosure of which patent is incorporated herein by reference.
As indicated, however, the most commonly used air heaters are those of the Ljungstrom type in which the drum is a cylindrical rotor containing metallic heat transfer plates, the rotor being movable with respect to a surrounding stationary housing. As the rotor turns, the heat transfer plates are first exposed to hot discharge gases, e.g. flue gases, and these heated plates then move into the air intake passage to heat the incoming air. The housing surrounding the drum includes sector plates which divide the housing into an air intake half and gas discharge half. To limit leakage between the drum and housing, the drum is typically provided with circumferentially extending, semi-rigid seal plates that are intended to pass closely by the annular wear bar in the housing with only a small clearance.
A major problem with the foregoing sealing arrangement is that it depends on achieving small, constant and predictable clearances between the seal plates and adjacent surfaces. Such clearances are difficult to attain even in a newly manufactured air preheater, and are particularly difficult to maintain in an air preheater that is in service. Air preheaters, when in service, are subject to extremes in temperature and are subject to a very hostile environments. Factors such as wear, distortion of parts due to temperature differentials, normal dimensional changes due to heating and cooling, irregular surfaces on the wear bars, out-of-roundness of the rotor and/or adjacent housing portion, and various other factors contribute, in practice, to wide variation in the clearances between the main elements which move relative to each other.
One particular problem is that the rotor and its seal plate tend to droop when heated, the hotter end of the rotor assuming a convex shape and the less hot end a concave shape. This, in turn, may result in excessive clearances at one end of the air heater and may also result in a complete lack of clearance at the other end, such there is an unintentional clashing of the metal seal plates with the adjacent sealing surfaces. These problems are further aggravated by the hostile environment to which an air preheater is subjected. The dirty, soot and acid-laden discharge gas which passes through the air preheater results in soot buildup, corrosion, and wear, all of which contribute to irregularities in the relatively movable parts. The irregularities, of course, lead to sealing difficulties.
As indicated, the circumferential seals which are original equipment in Ljungstrom type air heaters include semi-rigid plates mounted in close proximity to stationary sealing surfaces in the housing. These sealing surfaces, may, for example, be provided by annular wear bars mounted in the housing. Each stationary sealing surface expands and contracts with temperature changes, so its diameter changes along with the temperature changes. Thus, the clearance between the end of the semi-rigid seal plate and the sealing surface is subject to wide variation, resulting in significant leakage. The leakage is tantamount to a bypass around the air heater, and it reduces the efficiency of the plant. To control and minimize this leakage, the seals at the perimeter, i.e., the seals at the interface between the rotor and the housing, are the most critical of all the seals in terms of their effect upon overall plant performance.
Donald K. Hagar et al. U.S. Pat. Nos. 4,673,026 and 4,791,980, disclose radial and circumferential sealing arrangements which represent improvements over the semi-rigid radial and circumferential seal plates which are original equipment on Ljungstrom type air heaters. The present invention represents yet a further improvement in the circumferential sealing arrangement over that disclosed in the foregoing Hagar et al. patents, the disclosures of which are hereby incorporated herein by reference.
Circumferential seals, including the improved circumferential seals disclosed in the foregoing Hagar et al. patents have been the most service intensive of the seals in the air preheater. In view of their use of relatively thin strips of sealing material, the improved circumferential seals could be damaged when subject to severe conditions. Such improved seals left room for further improvement in terms of tolerance for out-of-roundness in the main, relatively rotatable elements of air heater. Despite the improvement they represented in reducing the chances of metal-to-metal clashing when normal clearances would disappear, such improved seals could still, under some circumstances, damage the surface with which they mated.
With these considerations in mind, the present invention seeks to provide a more efficient sealing mechanism which can be mounted on standard air heater structures, which structures may have main elements of significantly less than perfect cylindrical shape, i.e., air heater elements which have significant radial dimensional errors and which elements, during operation, may exhibit undesirable radial and axial displacements as well.
These undesirable radial and axial displacements occur both through shape and dimension changes caused by heating and cooling of the components of the air heater as well as basic, built-in location errors which are not necessarily temperature dependant. Thus, in addition to thermal displacements, it is not unusual to encounter significant built-in errors such as built-in out-of-roundness. Additionally, the thermal displacements result in a constantly changing radius for the rotor, making particularly challenging the task of providing a close tolerance seal, even with the improved circumferential seal disclosed in the foregoing Hagar patents.